Iron is an essential trace element for almost all organisms and in this context is relevant in particular for growth and blood formation. The balance of iron metabolism in this context is primarily regulated at the level of recovery of iron from haemoglobin from ageing erythrocytes and duodenal absorption of iron bonded in food. The iron released is absorbed via the intestine, in particular by way of specific transport systems (DMT-1, ferroportin, transferrin, transferrin receptors), transported into the blood stream and passed on by this means into the corresponding tissue and organs.
The element iron is of great importance in the human body inter alia for oxygen transport, oxygen uptake, cell functions, such as mitochondrial electron transport, and finally for energy metabolism in total.
The body of a human contains on average 4 to 5 g of iron, this being present in enzymes, in haemoglobin and myoglobin and as depot or reserve iron in the form of ferritin and haemosiderin.
About half of this iron, approx. 2 g, is present as haem iron bonded in the haemoglobin of red blood corpuscles. Since these erythrocytes have only a limited life (75-150 days), new ones must constantly be formed and old ones eliminated (over 2 million new erythrocytes are formed per second). This high regeneration capacity is achieved by macrophages, in that these absorb the ageing erythrocytes by phagocytosis, lyse them and in this way can recycle the iron contained in them for the iron metabolism. The amount of iron required daily for erythropoiesis of approx. 25 mg is thus mostly provided.
The daily iron requirement of an adult human is between 0.5 and 1.5 mg per day, and for infants and women in pregnancy the iron requirement is 2 to 5 mg per day. Daily iron losses, e.g. by exfoliation of skin cells and epithelial cells, is comparatively low, but increased iron losses occur, for example, in women during menstrual bleeding. Blood losses generally can considerably reduce iron metabolism, since about 1 mg of iron is lost per 2 ml of blood. The normal daily iron loss of approx. 1 mg is conventionally replaced again by an adult, healthy human via the daily food intake. Iron metabolism is regulated via absorption, the absorption rate of the iron present in food being between 6 and 12%, and in the event of iron deficiency the absorption rate is up to 25%. The absorption rate is regulated by the organism as a function of iron requirement and the size of the iron store. In this context, the human organism uses both divalent and trivalent iron ions. Iron(III) compounds are conventionally dissolved in the stomach at a sufficiently acid pH and are thus made available for absorption. Absorption of the iron takes place in the upper small intestine by mucosa cells. In this context, for absorption trivalent non-haem iron is first reduced to Fe2+ e.g. by ferrireductase (duodenal cytochrome b at the membrane) in the membrane of intestinal cells, so that it can then be transported by the transport protein DMT1 (divalent metal transporter 1) into the intestinal cells. On the other hand, haem iron enters into the enterocytes unchanged via the cell membrane. In the enterocytes, iron is either stored as depot iron in ferritin or released into the blood by the transport protein ferroportin, bonded to transferrin. Hepcidin plays a central role in this operation, since it is the essential regulation factor of iron uptake. The divalent iron transported into the blood by the ferroportin is converted into trivalent iron by oxidases (ceruloplasmin, hephaestin), which is then transported to the relevant places in the organism by means of transferrin (see for example: “Balancing acts: molecular control of mammalian iron metabolism”. M. W. Hentze, Cell 117, 2004, 285-297.)
The regulation of the iron level in this context is controlled or regulated by hepcidin.
Hepcidin is a peptide hormone which is produced in the liver. The prevailing active form has 25 amino acids (see for example: “Hepcidin, a key regulator of iron metabolism and mediator of anemia of inflammation”. T. Ganz Blood 102, 2003, 783-8), although two forms shortened at the amino end, hepcidin-22 and hepcidin-20, have been found. Hepcidin acts on iron uptake via the intestine, via the placenta and on the release of iron from the reticuloendothelial system. In the body, hepcidin is synthesized from so-called pro-hepcidin in the liver, pro-hepcidin being coded by the so-called HAMP gene. If the organism is adequately supplied with iron and oxygen, increased hepcidin is formed. In the mucosa cells of the small intestine and in the macrophages, hepcidin binds to ferroportin, by means of which iron is conventionally transported out of the cell interior into the blood.
The transport protein ferroportin is a membrane transport protein comprising 571 amino acids which is formed and located in the liver, spleen, kidneys, heart, intestine and placenta. In particular, in this context ferroportin is located in the basolateral membrane of intestinal epithelial cells. The ferroportin bound in this way effects export of iron into the blood here. In this context, ferroportin very probably transports iron as Fe2+. If hepcidin is bound to ferroportin, ferroportin is transported into the cell interior and degraded, as a result of which the release of iron from the cells is then almost completely blocked. If the ferroportin is inactivated via hepcidin, the iron stored in the mucosa cells therefore cannot be transported away, and the iron is lost with the natural exfoliation of cells via the stool. As a result, absorption of iron in the intestine is reduced by hepcidin. On the other hand, if the iron content in the serum is lowered, hepcidin production in the hepatocytes of the liver is reduced, so that less hepcidin is released and therefore less ferroportin is inactivated, as a result of which an increased amount of iron can be transported into the serum.
Ferroportin is moreover located to a high degree in the reticuloendothelial system (RES), to which the macrophages also belong.
Hepcidin plays an important role here in the event of impaired iron metabolism in the context of chronic inflammations, since interleukin-6 in particular is increased with such inflammations, which leads to an increase in the hepcidin level. Increased hepcidin is bound to the ferroportin of the macrophages by this means, as a result of which release of iron is blocked here, which in the end then leads to an inflammation-related anaemia (ACD or AI).
Since the organism of mammals cannot actively excrete iron, iron metabolism is essentially controlled via cellular release of iron from macrophages, hepatocytes and enterocytes by way of hepcidin.
Hepcidin thus plays an important role in functional anaemia. In this case, in spite of a full iron store, the iron requirement of bone marrow for erythropoiesis is not met sufficiently. The reason for this is assumed to be an increased hepcidin concentration, which in particular limits the transport of iron from the macrophages by blocking the ferroportin and thus greatly reduces the release of iron recycled by phagocytosis.
In the event of a disturbance in the hepcidin regulation mechanism, a direct effect thus manifests itself on iron metabolism in the organism. For example, if hepcidin expression is prevented, for example by a genetic defect, this leads directly to an overloading of iron, which is known as the iron storage disease haemochromatosis.
On the other hand, overexpression of hepcidin, for example due to inflammation processes, for example with chronic inflammations, results directly in reduced serum iron levels. In pathological cases this can lead to a reduced content of haemoglobin, reduced erythrocyte production and therefore to an anaemia.
The duration of use of chemotherapeutics in carcinoma treatments can be significantly reduced by an existing anaemia, since the state of reduced formation of red blood corpuscles caused by the chemotherapeutics employed is intensified still further by an existing anaemia.
Further symptoms of anaemias include tiredness, pallor and reduced attention capacities. The clinical symptoms of anaemia include low serum iron contents (hypoferraemia), low haemoglobin contents, low haematocrit level and a reduced number of red blood corpuscles, reduced reticulocytes and increased values of soluble transferrin receptors.
Iron deficiency symptoms or iron anaemias are conventionally treated by supplying iron. In this context, substitution with iron takes place either by the oral route or by intravenous administration of iron. Erythropoietin and other erythropoiesis-stimulating substances can moreover also be employed in the treatment of anaemias to give a boost to the formation of red blood corpuscles.
Anaemias which are caused by chronic diseases, e.g. chronic inflammatory diseases, can be treated only inadequately with such conventional treatment methods. Cytokines, such as in particular inflammatory cytokine, in particular play a particular role in anaemias which are based on chronic inflammation processes. An overexpression of hepcidin occurs in particular with such chronic inflammatory diseases and is known to lead to a reduced availability of iron for the formation of the red blood corpuscles.
From this emerges the need for an effective treatment method for hepcidin-mediated or -imparted anaemias, in particular those which cannot be treated with conventional iron substitution, such as those anaemias which are caused by chronic inflammatory diseases (ACD and AI).
Anaemia is to be attributed inter alia to those chronic inflammatory diseases mentioned, and to malnutrition or low-iron diets or unbalanced, low-iron eating habits. Anaemias moreover occur due to reduced or poor absorption of iron, for example due to gastrectomies or diseases such as Crohn's disease. An iron deficiency can also occur as a result of an increased blood loss, e.g. due to an injury, heavy menstrual bleeding or blood donation. An increased iron requirement in the growth phase of adolescents and children and in pregnant women is also known. Since an iron deficiency leads not only to a reduced formation of red blood corpuscles but therefore also to a poor supply of oxygen to the organism, which can lead to the abovementioned symptoms, such as tiredness, pallor and lack of concentration and also precisely in adolescents to long-term negative effects on cognitive development, a particularly effective therapy in addition to the known conventional substitution therapy is also of particular interest for this sector.
Compounds which bind to hepcidin or to ferroportin and therefore inhibit the binding of hepcidin to ferroportin and therefore in turn prevent the inactivation of ferroportin by hepcidin, or compounds which, although hepcidin is bound to ferroportin, prevent the internalization of the hepcidin-ferroportin complex, and in this manner prevent the inactivation of the ferroportin by the hepcidin, can be called in general terms hepcidin antagonists.
By using such hepcidin antagonists, there is moreover also generally the possibility, for example by inhibiting hepcidin expression or by blocking the hepcidin-ferroportin interaction, of acting directly on the regulation mechanism of hepcidin and therefore of preventing via this route blocking of the iron transport pathway from tissue macrophages, liver cells and mucosa cells into the serum via the transport protein ferroportin. With such hepcidin antagonists or ferroportin expression inhibitors, substances are therefore available which are suitable for the preparation of pharmaceutical compositions or medicaments in the treatment of anaemias, in particular anaemias with chronic inflammatory diseases. These substances can be employed for treatment of such disorders and the resulting diseases, since these have a direct influence on the increase in the release of recycled haem iron by macrophages and effect an increase in the iron absorption of iron released from food in the intestinal tract. Such substances, inhibitors of hepcidin expression or hepcidin antagonists, can therefore be used for treatment of iron metabolism disorders, such as iron deficiency diseases, anaemias and anaemia-related diseases. In particular, this also includes those anaemias which are caused by acute or chronic inflammatory diseases, such as, for example, osteoarticular diseases, such as rheumatoid polyarthritis, or diseases which are associated with inflammatory syndromes. Such substances can therefore be of particular benefit in particular in the indications of cancer, in particular colorectal cancer, multiple myeloma, ovarian and endometrial cancer and prostate cancer, CKD 3-5 (chronic kidney disease stage 3-5) CHF (chronic heart failure), RA (rheumatoid arthritis), SLE (systemic lupus erythematosus) and IBD (inflammatory bowel disease).